Steam generators of pressurized water nuclear reactors include tubes bent in a U having two straight parts or linear branches clamped at their ends in a tube plate. The linear branches of the tubes of the bundle are held are held by spacer plates arranged mutually parallel and distributed along the length of the linear branches and each including a network of through-openings for allowing passage of the tubes.
The networks of openings of the tube plate and of the spacer plates are identical and the openings are arranged in these networks so that the tubes of the bundle constitute parallel plane rows having a small spacing.
In each of the rows, the bends of substantially semicircular shape are juxtaposed and have radii of curvature which decrease from the outside towards the inside of the bundle. In addition, the bends of each of the rows which have the maximum radius of curvature have a radius which decreases from the central part towards the peripheral part of the bundle.
For this reason, the bent parts of the tubes constitute a substantially hemispherical shape called the tube bend region at the upper part of the steam generator bundle.
The tubes of a given row and the adjacent rows are separated by narrow free spaces allowing passage of feedwater of the steam generator inside the bundle, in contact with the outer exchanger surface of the tubes.
During operation of the steam generator, the pressurized water at high temperature circulates in the tubes of the bundle and the feedwater is brought into contact with the outer exchange surface of the tubes along which it moves in the vertical direction while being heated, and then vaporizes, to emerge in the form of steam in the upper part of the steam generator.
The circulation of the fluids in contact with the tubes may cause vibrations which can lead to damage of the tubes if they are not held efficiently.
The linear part of the tubes is held efficiently by rigid spacer plates. The curved parts of the tubes of the bundle constituting the tube bend region must also be held and antivibration bars which generally used for this purpose which are interposed between the adjacent tube rows of the bundle and arranged along substantially radial directions of the tube bend region. These spacer bars may be folded or assembled so as to have the shape of a V whose two branches are directed, in use, along radial directions of the tube bend region between the tube rows of each of the adjacent row pairs.
The bundle of the steam generator is assembled by successive rows, in the casing of the steam generator which is placed in the horizontal position on a turning gear.
Most of the operations of mounting and welding the elements of the steam generator are also carried out with the steam generator in a horizontal position.
During mounting of the bundle, the antivibration bars, which are introduced between the tube rows so that their branches have well-defined radial arrangements with respect to the tube bend region are capable of sliding and tilting, so that there is a risk that they will be fitted incorrectly.
At the end of the mounting of the bundle, the ends of the antivibration bars opposite their ends located in the central part of the tube bend region generally project with respect to the tubes which constitute the outer layer of the tube bend region and are connected together by curved bars resting on the outer surface of the tube bend region and on which the outer end parts of the antivibration bars are welded.
Other known modes of fastening the outer ends of the antivibration bars employ removable fastening elements such as screwed elements.
Insofar as the curved bars or other devices for holding the outer ends of the antivibration bars are fitted only after all the tubes of the bundle have been mounted, the antivibration bars are not held with respect to each other during the manufacturing phases subsequent to mounting of the bundle, for example during welding of the bottom of the steam generator constituting the water box or during final welding of the steam generator as a result tilting of the bars or other displacements can arise during the manufacture and lead to positioning defects of the antivibration bars and bending of the tube rows this, in turn leads to the presence of shape defects of the tube bend region of the steam generator.
Furthermore, during operation of the steam generator, the antivibration bars which are in contact with a fluid circulating at high speed may be displaced within the tube bend region, because they are connected together only by their outer ends and by linkage devices located above the surface of the tube bend region.
The spacer plates which hold the linear parts of the tubes of the steam generator are fixed to anchor rods which are perpendicular to the spacer plates and pass through them inside openings substituting for tube passage openings in the networks of the spacer plates.
Each of the anchor rods is fixed, by mechanical assembly parts to the upper spacer of the steam generator, i.e., to the spacer located closest to the bent part of the tubes of the bundle.
In extension of the anchor rods, inside the tube bend region, free spaces are left which are arranged between two successive tubes of one and the same row, because of the presence of the anchor rods within the bundle, in positions which might be occupied by tubes. The bundle of the steam generator therefore has certain discontinuities within the tube bend region, in extension of the anchor rods.
To date, no device has been known which makes it possible to mount the bundle of a steam generator while avoiding tilting or other displacement of the antivibration bars for holding the bent parts of the tubes, both during the manufacturing operations following the mounting of the tubes and during operation of the steam generator.